Fracturing subterranean formations using micellar dispersions

ABSTRACT

A subterranean formation is fractured by injecting under fracturing pressures a micellar dispersion comprised of hydrocarbon, aqueous medium, and surfactant. Propping agents are useful in the dispersion. Also, the viscosity of the dispersion is designed to obtain &#39;&#39;&#39;&#39;low penetrating fluids.

United States Patent [72] Inventor Marion 0. Son, Jr.

Littletorl, C010. [21] Appl. No. 20,099 [22] Filed Mar. 16, 1 970 [45 IPatented Sept. 7, 1971 [73] Assignee Marathon Oil Company Findlay, Ohio[54] FRACTURING SUBTERRANEAN FORMATIONS USING MICELLAR DISPERSIONS 19Claims, No Drawings [52] 11.8. CI 166/308, 252/855 R [51] int. Cl E21b43/26 [50] Field of Search 166/308, 274, 273, 250, 271, 281; 252/855 R[56] References Cited UNITED STATES PATENTS 2,802,531 8/1957 Cardwell eta1. 166/308 X Priest et a1.

Tosch et al. Davis, Jr. et al.

Kiel

Primary Examiner-Stephen 'J Novosad Att0rneys.|oseph C. Herring, RichardC. Willson, Jr. and

Jack L. Hummel ABSTRACT: A subterranean formation is fractured byinjecting under fracturing pressures a micellar dispersion comprised ofhydrocarbon, aqueous medium, and surfactant. Propping agents are usefulin the dispersion. Also, the Viscosity of the dispersion is designed toobtain low penetrating fluids."

FRACTURING SUBTERRANEAN FORMATIONS USING MICELLAR DISPERSIONS CROSSREFERENCE TO RELATED APPLICATION .U S. Pat. application Ser. No.762,141, filed Sept. 24, 1968, now U.S. Pat. No. 3,500,932, teaches theinjection of a micellar dispersion into a formation to be fracturedprevious to the injection of a fracturing fluid. The micellar dispersionis utilized as a prefrac fluid to clean the perforations, formation, andto minimize emulsion in the well bore area.

BACKGROUND OF THE INVENTION 1 Field of the Invention This inventionrelates to the hydraulic fracturing of subterranean hydrocarbon-bearingformations penetrated by a well.

2. Description of the Prior Art Hydrocarbon-bearing subterraneanformations often are contaminated with cement, drilling mud, foreignparticles, contain emulsion blocks, etc., or otherwise lack naturalformation pressure. Such can adversely affect the production ofhydrocarbon from the formation. One way of overcoming these adversitiesis to fracture the formation to facilitate the movement of hydrocarbonsvia permeable channels which are formed and which extend from theformation to the well bore.

U. S. Pat. No. 3,378,074 to Kiel teaches the use of water-inoilemulsions as fracturing agents and the use of a wetting agent to reducefriction between the injection string surface nd ract ge A problemattending traditional fracturing processes lies in the instability ofthe particular fracturing agent employed. Conventional fracturingagents, such as emulsions, must be substantially more viscous than waterfor most reservoirs before fractures can be obtained using reasonableinjection pressures. oftentimes, necessary fracturing pressures tend tolower the viscosity of the fracturing agent by molecular cleavage,emulsion breakdown, etc. These fracturing agents also tend to break downor otherwise suffer a decrease in a viscosity when transported to thewell site. As a result, production of the fracturing agent at the wellsite has been necessitated. Furthermore, storage of these fracturingfluids, whether at the well site, or elsewhere, has resulted in theirbreakdown due to their inherent instability.

These and other disadvantages of the prior art are overcome by utilizingthe novel fracturing agent and processes for its implementationaccording to the present invention.

DESCRIPTION OF THE INVENTION Briefly described, the invention comprisesa process for hydraulically fracturing a subterraneanhydrocarbon-bearing formation penetrated by at least one well. The novelfracturing agent is a micellar dispersion comprised of hydrocarbon,

aqueous media and surfactant (petroleum sulfonate) and having aviscosity of at least about cp. at ambient temperature, i.e. 22-23 C.The fracturing agent is conventionally injected into the formation (withor without the aid of a lubricating fluid to wet the inner surface ofthe injection tubing), under sufficient pressure to fracture theformation. The resulting fracture increases the permeability of theformation in the vicinity of the well bore. Propping agents may beincorporated in the fracturing fluid to hold open the fractures.

The term micellar dispersion as used herein is meant to include micellarsolutions and microemulsions. Examples of useful micellar dispersionsare included in US. Pat. Nos. 3,254,714, 3,275,075, 3,301,325,3,307,628, 3,330,344, 3,348,61 l, and 3,497,006. The micellar dispersioncan be oilexternal, or water-external, butJpreferably is oil-external.

The micellar dispersion is, for purposes of this invention, a relativelystable dispersion. By stable is meant that the dispersions arethermodynamically stable, appearing to be single phased andsubstantially transparent. Equilibrium tends toward further dispersionof the internal phase rather than coagulation or coalescence, the latteris characteristic of emulsions.

The micellar dispersion A is comprised of hydrocarbon, aqueous medium,and at least one surfactant. One or more cosurfactants (also identifiedas cosolvents, cosolubilizers, and semipolar organic compounds) areuseful, but not necessary, in the dispersions. Also, electrolytes areuseful in the dispersions. Also, the micellar dispersion can containother additives, e.g. corrosion and scale inhibitors, bactericides, etc.Ex-

amples of useful dispersions include those containing, by

volume, from about 4 percent to about percent hydrocarbon; from about 1to about 70 percent or more of water, at least about 4 percentsurfactant, from about 0.01 to about 20 percent or more by volume ofcosurfactant and from about 0.001 or less to about 5 percent or more byweight of electrolyte in the aqueous phase.

Examples of useful hydrocarbons include crude oil, partially refinedfractions thereof, e.g. side cuts from crude columns, crude columnoverheads, gas oils, kerosene, heavy naphthas, naphthas, straight-rungasoline, and liquefied petroleum gases, refined fractions of crude oiland halogenated hydrocarbons. Pure hydrocarbons are also useful, e.g.paraffinic compounds including liquefied petroleum gases, propane,pentane, heptane, decane, dodecane, etc.; cycloparaffin compoundsincluding cyclohexane, etc.; aryl compounds including monocyclic andpolycyclic and substituted products thereof including toluene, alkylphenols, etc. and combinations of the hydrocarbons taught herein. Basedon economics and viscosity considerations, the preferred hydrocarbon isone locally available and is crude oil. The unsulfonated hydrocarbon (e.g. heavy vacuum gas oils) in petroleum sulfonates is also useful.

The aqueous medium can be soft, brackish, or brine water. Preferably,the water is soft but it can contain small amounts of salts.

Examples of surfactants can be found in US. Pat. No. 3,254,714 toGogarty et a1. Preferably, the surfactant is a petroleum sulfonate, alsoknown as alkaryl sulfonates or alkaryl naphthenic sulfonates. Thesulfonate can contain less than 60 and up to percent active sulfonate.Examples of preferred surfactants are the sodium and ammonium petrole umsulfonates. The sulfonates can have an average equivalent weight withinthe range of from about 350 to about 520, and more preferably from about350 to about 470. By equivalent weight is meant the molecular weightdivided by the number of sulfonate groups attached to the sulfonatemolecule. The surfactant can be a mixture of low, medium and highaverage equivalent weight sulfonates or a mixture of differentsurfactants.

Examples of useful cosurfactants include alcohols, amino compounds,esters, aldehydes and ketones containing from 1 up to about 20 or morecarbon atoms and more preferably from about 3 to about 16 carbon atoms.The cosurfactant is preferably an alcohol, e.g. isopropanol, nandisobutanol, the amyl alcohols such as n-amyl alcohol, land 2-hexanol,land 2-octanol, decyl alcohols, alkaryl alcohols such as p-nonyl phenol,alcoholic liquors such as fusel oil, hydroxy compounds such as2-butoxychanol, and like compounds. Particularly useful alcohols includethe primary butanols, primary pentanols and secondary hexanols.Concentrations of from about 0.1 percent to more than about 10 percentby volume are preferred and more preferably from about 0.2 percent toabout 3 percent. Mixtures of two or more cosurfactants are also useful.

Electrolytes useful within the micellar dispersions include inorganicbases, inorganic acids, inorganic salts, organic bases, organic acids,and organic salts. The electrolytes can be strongly or weakly ionized.Preferably, the electrolytes are inorganic bases, inorganic acids andinorganic salts, examples include sodium hydroxide sodium chloride,sodium sulfate, hydrochloric acid, sulfuric acid, sodium nitrate,ammonium chloride, ammonium hydroxide, and potassium chloride. Examplesof other useful electrolytes can'be found in U.S. Pat. No. 3,330,344.The type and concentration of preferred electrolyte will depend onphase, hydrocarbon phase, aqueous phase, surfactant, cosurfactant,operating conditions (eg temperature) compatibility with proppingagents, etc.

The components of the micellar dispersion are admixed with agitation inany table manner, such as by stirring, shaking rotary stirring, Apumping to form a stable micellar dispersion. This micellar dispersionmay be prepared at the well site, or elsewhere. The dispersion can bedesigned not to phase separate upon storage under wide temperaturefluctuations.

The viscosity of the micellar dispersion fracturing agent may be variedover a wide range. The desired viscosity is selected by considering theparticular well and formation to be treated. important considerationsinclude permeability of the formation, presence or absence ofinterstitial water, availability of natural brine to use as the aqueouscomponent of the fracturing agent, diameter and friction properties ofthe injection string, etc. Generally, the viscosity of the fracturingagent can vary directly with the permeability of the formation; thus,for highly permeable formations, a high viscosity fracturing fluid isgenerally necessary bproduce sizeable fractures.

The characteristics of the micellar dispersion, especially viscosity,may be tailored to fit the particular reservoir The viscosity of thefracturing agent may also be monitored at the surface to account for anyin situ reservoir changes which would require a redesigning of themicellar dispersion. Viscosity control, in general, is obtained byvarying a number of parameters, including water concentration,particular hydrocarbon, particular sulfonate, cosurfactant (if any),electrolyte (if any) and control of the hydrophile lipophile balance(HLB).

One of the most important parameters for control of viscosity is theaqueous medium concentration of the micellar dispersion. For preferredviscosities of at least 10 cp. at ambient temperature, and preferably inthe range of from about 100 to about 2,000 cp. at ambient temperature,depending on the characteristics of the reservoir, the waterconcentration should preferably be from about i to about 50 and morepreferably from out 2 to about 25 percent by volume. The general shapeof the curve defining the relationship between viscosity and waterconcentration is depicted in the drawing of US. Pat. No. 3,254,714 toGogarty. This curve demonstrates that the viscosity can go to a maximumat a relatively low concentration of water. At or near this low waterconcentration, the viscosity of the fracturing agent may be variedwithin a wide degree by merely monitoring the water concentration withina relatively small range of concentrations. The curve may be shiftedwith respect to the horizontal and/or vertical axis and also variedsomewhat in shape depending on the particular system involved Factorswhich affect the disposition of the curve (and hence properties of thesystem) include molecular weight of surfactants, type of hydrocarbon,presence of cosurfactant and presence of electrolyte.

Viscosity control of the fracturing agent is within the skill of thosefamiliar with micellar dispersion design.

Any of a number of commonly employed propping agents may be incorporatedwithin the fracturing fluid and injected into the formation. The higherthe viscosity of the fracturing agent, the greater its capacity to carrylarger and heavier propping material. Propping agents useful for thepresent invention include coarse and grains, tempered glass beads,rounded walnut shell fragments, aluminum pellets, and similar materials.Such agents are generally used in concentrations between about 0.1 andabout 5 lbs/gallon and preferably about 0.5-3.5 lbs/gallon of fracturingagent. in general, these propping agents with particle sizes of 6 meshare to about 400, more preferably 10 to about 100 and most preferably 10to about mesh are employed, depending on the particular reservoir andwell to be treated.

One advantage contemplated by the invention is that friction between theinjection string and the fracturing fluid (with or without proppingagent) will be relatively low, with the result that it is not normallynecessary to lubricate the injection string to permit greater injectionrates. The micellar dispersions of the present invention tend to beself-lubricating. However, it may be desirable in certain casesespecially where extremely high viscosity fluids are to be injected, tocoat the injecting string with a material compatible with the micellardispersion. Since the micellar dispersions are substantially oilexternalsystems, the lubricating fluid is preferably one which is predominantlymiscible with be hydrocarbon phase of the micellar dispersion. Examplesof useful lubricating fluids include those hydrocarbons describedhereinabove as examples of components of of the micellar dispersion.More preferably, aqueous polymer solutions, exemplified by aqueoussolutions of partially hydrolyzed, high molecular weightpolyacrylamides, polysaccharides, polyethylene oxides carbon methylcellulose, carboxy vinyl polymers, and solutions oflike materials areemployed.

A slug of mobility buffer may follow the injected slug of fracturingfluid This mobility buffer may in turn be followed by a drive fluid ofcompatible mobility with the mobility buffer. By mobility buffer ismeant a fluid containing a mobility reducing agent and preferably havinga mobility about equal to or greater than the mobility of the fracturingfluid. Preferably, the mobility of the buffer is graded so that thefront of it has a mobility about equal to or slightly greater than theback portion of the fracturing fluid. A higher mobility can becharacteristic of the trailing edge of the buffer, preferably aboutequal to or slightly less than the mobility of the following drivefluid, if any. Preferred mobility buffers are the lubricating fluidslisted hereinabove, and especially preferred are aqueous solutions ofpartially hydrolyzed polyacrylamides. A number of drive fluids may beemployed, exemplified by brine, water, thickened water, straight-rungasoline, and LPG.

The rates of injection should be sufficiently high to increase thepressure above the fracturing pressure (overburden pressure). Theserates will depend upon downhole pressure, permeability of the formation,width of the formation, etc. For most reservoirs, any rate of injectionwill be sufficient if such imparts a downhole pressure between about 500and about 5,000 p.s.i.

Prefracturing agents may be injected into the reservoir head of themicellar dispersion, such as acids or surfactant solutions, although ingeneral the fracturing agents of the present invention tend toself-clean the well bore and formation. Also, diverting agents may beintermittently injected to obtain a more uniform fracturing profile-suchare recommended in reservoirs containing highly permeable zones.

The following examples specifically illustrate micellar dispersionsuseful with this invention. Unless otherwise specified, all percents arebased on volume. The viscosities of the micellar dispersions aremeasured at 72 F. On a Brookfield Viscometer. These dispersions areobtained by mixing the surfactant and hydrocarbon and then adding waterto obtain the indicated viscosities. Compositions of the micellardispersions are indicated in Table 1:

TABLE I Viscosity (cps.) at Sample No. Composition of micellardispersion 72 F.

7.5 g. of Shell sulfonate A as 5 2.5 g. of Pyronate 50 96 mi.hydrocarbon (SRG +VO 25 ml. distilled water 7.5 g. Petronate KL. B 2.5g. Pyronate 50 96 ml. hydrocarbon (SRG-l-VO) 25 ml. distilled water {7g. Shell sulfonate C 3 g. Pyronate 50 96 ml. hydrocarbon (SRG-l-VO) 25ml. distilled water 7 g. D 3 g. Pyronate 50 {5.5 g. Shell sultonate E4.5 g. Pyronate 50 96 ml. hydrocarbon (SRG-l-VO) 25 ml. distilled water10 g. Pyronate 50 F 96 ml. hydrocarbon (SRG-l-VO) 25 ml. distilled water1 Shell sulfonate, sodium petroleum sullonate. marketed by ShellChemical Co. 100% active slnce V0 has been extracted, average equivalentWeight 460-465.

The invention is 'notintended' to'belimited'by theforegoing description,rather, all modifications and equivalents obvious to those skilled inthe art are intended to be included within the scope of the invention astaught within the specification and appended claims.

What is claimed is:

l. A process of hydraulically fracturing a subterraneanhydrocarbon-bearing formation penetrated by at least one wall comprisinginjecting into the formation, at a pressure sufficient to fracture theformation, a micellar dispersion comprised of hydrocarbon, aqueousmedium and surfactant and having a viscosity of at least about tocentipoises at ambient temperature, thereby fracturing said formation.

by additionally incorporating a selected amount of cosurfactant into themicellar dispersion.

7. The process of claim 4 wherein the viscosity is monitored byadditionally incorporating a selected amount of electrolyte into themicellar dispersions.

8. The process of claim 1 wherein the viscosity of the mice]- lardispersion is between about 100 and about 2,000 centipoiscs at ambienttemperature.

9. The process of claim 1 wherein the injection pressure of the micellardispersion is within the range or about 500 to about 5,000 p.s.i.

10. The process of claim 1 wherein the aqueous medium concentrationwithin the micellar dispersion is about 1 to about 70 percent by volume.

11. The process of claim 1 wherein %e aqueous medium concentrationwithin the micellar dispersion is about 1 to about SO'pcrcent by volume.

12. The process of claim 1 wherein the hydrocarbon is crude oil or apartially refined fraction of crude oil.

13. The process of claim 1 wherein the surfactant is an alkylarylnaphthenic sulfonate having an average equivalent weight of about 350 toabout 520.

14. A process of hydraulically fracturing a subterraneanhydrocarbon-bearing formation penetrated by at least one injection meanscomprising injecting into the formation at a pressure sufficient tofracture the formation, a micellar dispersion comprised of hydrocarbon,about 1 to about 50 percent by volume aqueous medium at least 4 percentsurfactant having an average equivalent weight within the range of about350 to about 520 and the micellar dispersion having a viscosity of atleast about 10 cp. at ambient temperature, thereby fracturing theformation.

15. The process of claim 14 wherein the equivalent weight of thesulfonate is within the range of about 90 to about 460.

16. The process of claim 14 wherein the micellar dispersion containscosurfactant and/or electrolyte.

17. The process of claim 14 wherein the micellar dispersion containspropping agents.

18. The proccsswof claim'17whcrcin the-.miccllar dispersion 5conta'insaboutfOQl to'aboutj'llbs. of:thc-propping agent per gallon ofthe'miccllar dispersion V i W 19. The process of claim 14 whcreinthemiccllar dispersion is designed to have a viscosity within the range ofabout to about 2,000 cp. at ambient temperature.

Patent No.

Inventor(s) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION DatedSept. 7, 1971 Marion 0. ggn, Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

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After "about" insert l0 After "dispersion" delete --A:-.

Delete "2-butoxychanol," and insert 2butoxyethanol.

Delete "table" -suitable.

and insert After "stirring, Delete "A" and insert or.

Delete "bproduce" and insert to produce-.

insert Delete "wall" and insert -well--.

Delete "%e" and insert -the.

Signed and sealed this 1mm day of March 1972;

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(SEAL) Attest:

EDWARD M.FLETCHER, JR. Attesting Officer ROBERT GOTTSCHALK Commissionerof Patents

2. The process of claim 1 wherein the surfactant is petroleum sulfonate.3. The process of claim 1 wherein propping agent is incorporated withinthe micellar dispersion.
 4. The process of claim 1 wherein the viscosityof the micellar dispersion is monitored by varying the relativeconcentration of hydrocarbons, aqueous medium, and/or surfactant of themicellar dispersion.
 5. The process of claim 4 wherein the viscosity ismonitored by varying the selection of particular hydrocarbon, aqueousmedium or surfactant employed.
 6. The process of claim 4 wherein theviscosity is monitored by additionally incorporating a selected amountof cosurfactant into the micellar dispersion.
 7. The process of claim 4wherein the viscosity is monitored by additionally incorporating aselected amount of electrolyte into the micellar dispersions.
 8. Theprocess of claim 1 wherein the viscosity of the micellar dispersion isbetween about 100 and about 2,000 centipoises at ambient temperature. 9.The process of claim 1 wherein the injection pressure of the micellardispersion is within the range of about 500 to about 5, 000 p.s.i. 10.The process of claim 1 wherein the aqueous medium concentration withinthe micellar dispersion is about 1 to about 70 percent by volume. 11.The process of claim 1 wherein e aqueous medium concentration within themicellar dispersion is about 1 to about 50 percent by volume.
 12. Theprocess of claim 1 wherein the hydrocarbon is crude oil or a partiallyrefined fraction of crude oil.
 13. The process of claim 1 wherein thesurfactant is an alkylaryl naphthenic sulfonate having an averageequivalent weight of about 350 to about
 520. 14. A process ofhydraulically fracturing a subterranean hydrocarbon-bearing formationpenetrated by at least one injection means comprising injecting into theformation at a pressure sufficient to fracture the formation, a micellardispersion comprised of hydrocarbon, about 1 to about 50 percent byvolume aqueous medium at least 4 percent surfactant having an averageequivalent weight within the range of about 350 to about 520 and themicellar disPersion having a viscosity of at least about 10 cp. atambient temperature, thereby fracturing the formation.
 15. The processof claim 14 wherein the equivalent weight of the sulfonate is within therange of about 90 to about
 460. 16. The process of claim 14 wherein themicellar dispersion contains cosurfactant and/or electrolyte.
 17. Theprocess of claim 14 wherein the micellar dispersion contains proppingagents.
 18. The process of claim 17 wherein the micellar dispersioncontains about 0.1 to about 5 lbs. of the propping agent per gallon ofthe micellar dispersion.
 19. The process of claim 14 wherein themicellar dispersion is designed to have a viscosity within the range ofabout 100 to about 2,000 cp. at ambient temperature.